1. Field of the Invention
The present invention relates to a vehicle mounted engine control apparatus, and in particular to an improvement in a control technique for heaters for activating exhaust gas sensors used for air fuel ratio control in a vehicle mounted engine at an early stage.
2. Description of the Related Art
In general, a vehicle mounted engine control apparatus is provided with a fuel injection control unit that controls an amount of fuel to be injected so as to obtain a target air fuel ratio by controlling a supply amount of fuel to be injected by an injector (an amount of injection fuel) in proportion to the amount of intake air detected by an air flow sensor. In addition, the vehicle mounted engine control apparatus is constructed in such a manner that the control characteristic of the air fuel ratio in the fuel injection control unit is corrected by the use of a pair of exhaust gas sensors which are arranged at locations upstream and downstream, respectively, of a catalyzer which is arranged in an exhaust passage of the internal combustion engine for removing harmful substances in an exhaust gas. Various contrivances have been made with respect to a heater control method for activating the pair of exhaust gas sensors at an early stage (for example, see a first patent document, a second patent document, and a third patent document).
According to a heater control apparatus for an air fuel ratio sensor described in the first patent document, as a heater driving requirement for reducing the electric power consumption of the upstream (front) heater and the downstream (rear) heater, for example, when the rotational speed of the internal combustion engine is 500 rpm or less or the temperature of engine cooling water is 15 degrees C. or less, it is determined that heating by means of the heaters is untimely or too early, and electric power is not supplied to any of the upstream and downstream heaters.
In addition, if the rotational speed of the engine is equal to or greater than 2,500 rpm with the cooling water temperature of the internal combustion engine being equal to or higher than 30 degrees C., or if the engine rotational speed is equal to or greater than 4,500 rpm even with the cooling water temperature being in the range of 15 through 30 degrees C. (i.e., 30 degrees C. or less), it is determined that heating by means of the heaters has already been unnecessary, and electric power is not supplied to any of the upstream and downstream heaters.
However, in cases where the rotational speed and the environmental temperature of the internal combustion engine are in a predetermined range, for example, when the engine rotational speed is 2,000 rpm or less with the cooling water temperature of the internal combustion engine being equal to or higher than 30 degrees C., or when the engine rotational speed is 4,500 rpm or less with the cooling water temperature of the internal combustion engine being in the range of 15 through 30 degrees C., electric power is supplied to both of the upstream and downstream heaters.
In addition, attention is paid to the downstream heater in which the temperature of the exhaust gas is low, and if the engine rotational speed is in the range of 2,000 through 2,500 rpm with the cooling water temperature of the internal combustion engine being equal to or higher than 30 degrees C., the supply of electric power to the upstream heater is stopped, but the supply of electric power to the downstream heater is carried out.
On the other hand, according to a control apparatus for an internal combustion engine described in the second patent document, by supplying electric power to the downstream heater at a time later than that at which electric power is supplied to the upper heater, The battery stage fright which prevents the element cracks of the downstream heater to which the moisture adheres easily according to simultaneous electric supply at the same time is prevented.
Further, according to an air fuel ratio control apparatus for an internal combustion engine described in the third patent document, a first air fuel ratio control unit, which is operated in response to an upstream oxygen concentration sensor, and a second air fuel ratio control unit, which is operated in response to a downstream oxygen concentration sensor, are arranged in a subordinate manner so that the air fuel ratio is controlled by an injector driving unit.